We've seen several small arms weapons so far, as well as vehicle mounted ones. Obviously, we won't know until the game is released, and maybe not even then. However, we do know that there will be a mix of ranged and melee weapons, all of which will be realistic to the point of recoiling gun parts and visibly depleting ammunition. However, over numerous previews and screenshots, we've do know that humans will have at least these weapons:
 

A lot of the Marathon classics there, and we have seen several variations of the same weapon, notably the pistol. In addition to these, the aliens will also have several weapons, though their exact natures are often unclear:

It is also known at this point that players will only be able to carry two weapons at a time.